of Exceptional Stability as Regards
Climate._--It will be seen, by a reference to the diagram at page 171, that
during the last three million years the excentricity has been _less_ than
it is now on eight occasions, for short periods only, making up a total of
about 280,000 years; while it has been _more_ than it is now for many long
periods, of from 300,000 to 700,000 years each, making a total of 2,720,000
years; or nearly as 10 to 1. For nearly half the entire period, or
1,400,000 years, the excentricity has been nearly double what it is now,
and this is not far from its mean condition. We have no reason for
supposing that this long period of three million years, for which we have
tables, was in any way exceptional as regards the degree or variation of
excentricity; but, on the contrary, we may pretty safely assume that its
variations during this time fairly represent its average state of increase
and decrease during all known geological time. But when the glacial epoch
ended, 72,000 years ago, the excentricity was about double its present
amount; it then rapidly decreased till, at 60,000 years back, it was very
little greater than it is now, and since then it has been uniformly small.
It follows that, for about 60,000 years before our time, the mutations of
climate every 10,500 years have been comparatively unimportant, and that
the temperate zones have enjoyed _an exceptional stability of climate_.
During this time those powerful causes of organic change which depend on
considerable changes of climate and the consequent modifications,
migrations, and extinctions of species, will not have been at work; the
slight changes that did occur would probably be so slow and so little
marked that the various species would be able to adapt themselves to them
without much disturbance; and the result would be _an epoch of exceptional
stability of species_.

But it is from this very period of _exceptional stability_ that we obtain
our only _scale_ for measuring the rate of organic change. It includes not
only the historical period, {122} but that of the Swiss Lake dwellings, the
Danish shell-mounds, our peat-bogs, our sunken forests, and many of our
superficial alluvial deposits--the whole in fact, of the iron, bronze, and
neolithic ages. Even some portion of the palaeolithic age, and of the more
recent gravels and cave-earths may come into the same general period if
they were formed when the glacial epoch was passing away. Now th